[kbn/config-schema] Improve OneOf error message format

[mshustov]

The current error message for the failed .oneOf method is cryptic and not user friendly:

kibana/x-pack/plugins/security/server/config.test.ts

Lines 992 to 1000 in 1cc1d8c

	).toThrowErrorMatchingInlineSnapshot(`
	"[authc.providers]: types that failed validation:
	- [authc.providers.0]: expected value of type [array] but got [Object]
	- [authc.providers.1.anonymous.anonymous1.credentials]: types that failed validation:
	- [credentials.0.username]: expected value of type [string] but got [undefined]
	- [credentials.1.apiKey]: types that failed validation:
	- [credentials.apiKey.0.id]: expected value of type [string] but got [undefined]
	- [credentials.apiKey.1]: expected value of type [string] but got [Object]"
	`);

It's hard to reason about it. While the error doesn't provide details of the valid format (see https://github.com/elastic/kibana/blob/e3ca8a928dbb5fe2b844497a62f8875c8656deb5/x-pack/plugins/security/server/config.ts):

• xpack.securuty.authc.providers is expected to be string[] | { [key]:{ description: string, icon: string, showInSelector: string }
• xpack.securuty.authc.providers.anonymous.anonymous1.credentials is expected to be {username: string, password:string} | { apiKey: { id:string, key:string} | string } }

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)